I think that it depends on what experience that you want. We have been on Thanksgiving and leading up to Christmas. I really think that crowds are an unknown because of school schedules, vacation schedules and folks who just want to go. I do know that there is a BB that goes from December 9 thru the 23rd I believe, so I think that will impact when folks go in December. I would go when it makes best sense for my family and make my plans accordingly.
From my own experience, and this is mine alone, as pretty as it was Thanksgiving and the week following, I just was not all that Christmassy. We did all the events and shows, and it was cold, so that was not the issue. We just had not moved from the Thanksgiving mindset to the all out Christmas spirit. I now wait until December if I want Holidays in Disney, and have my house and shopping done, so when I get to Disney we are full out Holiday.